1. Field of the Invention
This invention relates generally to the cannulation of veins and arteries under the guidance of ultrasound.
2. State of the Art
Insertion of catheters into central veins or arteries can be a difficult task because the vein or artery may be located deep within the body or may otherwise be difficult to access in a particular patient. Multiple attempts at penetration may result in extreme discomfort to the patient and loss of valuable time during emergency situations. Furthermore, central veins and arteries are often in close proximity to each other. While attempting to access the internal jugular vein, for example, the carotid artery may instead be punctured, resulting in severe complications or even mortality due to consequent blood loss due to the high pressure of the blood flowing in the artery.
To prevent complications during catheterization, it is known that ultrasonic instruments can be used to determine the location and direction of the vessel to be penetrated. Various approaches use a Doppler-only technique with no imaging. One such technique transmits ultrasonic waves via a transducer from the skin surface to the vessel. Due to the flow of blood in the vessel, or the pulsation of the vascular walls, the ultrasonic wave undergoes a Doppler shift effect, which causes the reflected signal to be at a frequency different from the transmitted signal. The frequency difference between the transmitted and received signals is then converted to an electrical signal, amplified and sent to an audio speaker. The frequency of the tone emitted from the speaker increases as the frequency difference becomes greater, indicating the approximate location of the vessel. Improvements to this technique place either the transmitting transducer, receiving transducer, or both transmitting and receiving transducers within a hollow needle, so that the audio signal becomes louder as the needle is turned towards a vessel within the patient's body. While such applications are helpful in guiding the needle towards the general location of vessels, the obtainable accuracy is obviously limited. Other limitations of this technology include difficulty distinguishing veins from nearby arteries, difficulty determining when the vessel has been penetrated, and difficulty implementing the known Seldinger technique.
Other conventional approaches to identification of vessel location and direction use two-dimensional ultrasound imaging to either mark the vessel location on the skin before attempting to access the vessel using the known Seldinger technique or view the vessel as the needle tip advances towards it. See British Journal of Anaesthesia, 822–6 (1999). However, it would be desirable to improve ultrasound imaging techniques for the cannulation of blood vessels to make the use of such technology less cumbersome and more accurate.